Hello Matthew,
In your situation, I would definitely suggest a roof mounted antenna. Your pathways to Seattle (magnetic 352-357) already has 1 and 2 edge pathways. Asking those weakened signals to get through your roof is a recipe for unreliable reception.
Another complicating factor is that with any attic mount requiring a signal to pass through your roof knocks about 1/3 out of it. In your case those Seattle signals are trending towards weak. As a general rule, when you get below 25db in strength on a signal in an attic install, you may not see the signal at all due to not being able to "punch" through the roof.
All that said, if I were you, I'd do a single antenna solution. An
Antennacraft HBU33 would be a starting point.
http://www.solidsignal.com/pview.asp...na-%28hbu33%29 Orientate to magnetic heading 330. Make sure you have as unobstructed pathway as possible.
How many TVs do you intend to feed?
Cheers.